Plant growth-promoting bacteria (PGPB) are associated with many, if not all, plant species and are commonly present in many environments. The most widely studied group of PGPB is plant growth-promoting rhizobacteria (PGPR), which colonize the root surfaces and the closely adhering soil interface, the rhizosphere. Inside the rhizosphere is a zone where bacteria, fungi, and other organisms compete for nutrients and for binding to the root structures of the plant. Both detrimental and beneficial bacteria can occupy the plant's roots. The presence of plant growth-promoting bacteria (PGPB) within or near the roots or seeds can lead to a healthier rhizosphere environment and healthier plants. These free living bacteria promote plant growth in agricultural crops and lead to increased growth and yield at harvest. The bacteria that colonize the roots and maintain their benefits throughout the growth cycle of the plant are especially desired for application during early growth or as a seed coating agent to agricultural crops.
The mechanisms that PGPBs use in promoting plant growth are diverse and often plant- or cultivar-specific. Several PGPB growth-promoting mechanisms are known, which can influence the plant in a direct or indirect manner. The direct mechanism involves increasing plant growth by supplying the plant with nutrients and hormones, such as by fixing nitrogen that is available to plants, synthesizing phytohormones, and providing nutrients such as phosphate to the plant. The indirect mechanism of action for PGPB occurs through the ability to control detrimental fungal and bacterial pathogens from establishing or surviving within the rhizosphere. This is usually achieved through the beneficial secretion of antifungals and other antibiotics by the PGPB. As an additional advantage, PGPBs can also lead to extensive remodeling of the plant root systems.
In recent years, a significant effort has been expanded to identify novel strains of plant growth-promoting bacteria, and use them to promote plant growth, thereby increasing the yield of plant product, reducing the use and amounts of fertilizers and herbicides, and providing other benefits for agricultural and horticultural communities.